In oil and gas rotary drilling operations, an operator or contractor may desire to use downhole equipment to monitor subsurface conditions in a drill string used to drill a well. Various types of downhole equipment may be used to measure deviation of the well, resistivity of subsurface formations, or the pressure or temperature in the wellbore. To transmit high speed electrical signals from the downhole equipment to instrumentation at the surface, a wellbore telemetry cable can be installed in the drill string. Typically, the wellbore telemetry cable is an insulated single-conductor or multi-conductor cable armored with wire.
In early wellbore telemetry operations, a single continuous cable was initially installed between the drilling rig and the downhole equipment. One end of the cable was lowered into the drill string and was attached to downhole equipment at a subsurface location in the drill string. The other end of the cable was connected to a terminal at the surface. To advance the depth of the well, the entire cable had to be withdrawn from the drill string before each pipe section could be added to extend the length of the drill string. Because the addition of each new pipe section to the drill string required each cable to be replaced with a longer cable, the telemetry operations required a large stock of cables of varying lengths. To reduce the number of cables which were required to drill a well to a desired depth, a long cable would be installed in the drill string so that the cable was initially slackened. As drill pipe sections were added to advance the depth of the well, slack in the cable would gradually be reduced by an amount corresponding to the length of each added drill pipe section. This approach was not satisfactory because the slackened cable would tend to foul before the slack in the cable could be removed.
To reduce the need for a large supply of wellbore telemetry cables, U.S. Pat. No. 3,807,502 to Heilhecker et al. disclosed a technique for adding a new cable section to extend the length of the telemetry cable as each new pipe section was added to the drill string. Initially, a long cable was suspended from a structural support, called a spider, which was connected to the upper end of the drill string. The lower end of a cable section, which was threaded through a new drill pipe section, was connected to the upper end of the long telemetry cable. The drill pipe section was then added to lengthen the drill string, the depth of the well was advanced, and a new cable section and drill pipe section were added. Because the length of each cable section added to the telemetry cable was greater than the length of the corresponding drill pipe section added to the drill string, excess cable would accumulate in the drill string and would tend to foul. To remove the excess slack from the cable, portions of the cable were overlapped and were clamped with metal bindings.
Various techniques have been developed to store excess cable in the drill string by overlapping lengths of the cable. For example, U.S. Pat. No. 3,825,078 to Heilhecker et al. and U.S. Pat. No. 3,913,688 to Heilhecker et al. each disclosed a cable having its lower end attached to a subsurface location. Each cable was reeved around an upper sheave attached to the inside wall of the drill string and was reeved around a lower sheave to form an overlapping loop in the cable. The upper end of the cable was then connected to a surface terminal. A weight attached to the lower sheave prevented excess slack from accumulating in the cable.
A disadvantage of the overlapped cable configuration was that the lower sheave and overlapping portion of the cable tended to foul as drilling mud was pumped through the drill string. To reduce tangling of the cable with the lower sheave, U.S. Pat. No. 3,957,118 to Barry et al. disclosed a cable gripping device attached to the upper pulley for tensioning the portion of the cable between the upper pulley and the lower, subsurface location. The tensioned portion of the cable was used as a guide for controlling the travel of the lower pulley. Although the concept of guiding the lower pulley was improved in U.S. Pat. No. 4,098,342 to Robinson et al. and in U.S. Pat. No. 4,271,908 to Robinson, et al., these improved wire-guided loop systems do not completely eliminate fouling of the cable.
Although improved loop systems have been developed to store excess cable in the drill string without fouling the cable, loop systems have several distinct limitations which may be intolerable in commercial drilling operations. First, the drill string cannot be rotated or reciprocated during installation of a loop system. This limitation is undesirable because operators and contractors prefer to continuously move the drill string to lessen the chance of differential-pressure sticking between the drill pipe and the borehole wall. Second, existing loop systems may not be readily retrievable from the drill string in the event that the recovery mechanism should malfunction. In such event, the entire drill string may have to be tripped out of the borehole. If a portion of the loop system should be left in the drill string in an emergency situation, well control efforts could be hindered.
Accordingly, a need exists for a telemetry system which can be modified as the drill string penetrates deeper into the subsurface formations so that slack does not accumulate in the cable. Furthermore, the telemetry system should be capable of being installed during rotation and reciprocation of the drill string and should be fully recoverable from the drill string.